The Cell Membrane

Ms. Napolitano & Mrs. HaasCP Biology

The Cell Membrane Thin, flexible barrier that surrounds

the cell

AKA plasma membrane

Selectively permeable – allows some substances to cross more easily than others

Usually ~8nm thick

The Phospholipid Bilayer

Fluid Mosaic Model Used to describe the structure of the membrane

Fluid structure of phospholipids with a mosaic of various proteins embedded in it

Cholesterol helps maintain fluidity

Membrane becomes less fluid as temperature increases by restricting movement

Membrane becomes more fluid as temperature decreases by maintaining space

Structure of the Membrane

Membrane Proteins Different types of cells contain different types of

membrane proteins

Integral proteins – embedded into the hydrophobic core of the lipid bilayer

May or may not go all the way through the membrane

Peripheral proteins – on the bilayer surface

Integral Peripheral

Types of Membrane Proteins(1 of 2)

1. Transport Channel or carrier proteins (hydrophilic

tunnels) allow polar molecules and ions to pass through the hydrophobic layer

Proteins are specific for the substance they transport

2. Enzymatic

3. Signal Transduction (chemical messages) Receptor proteins transmit information from

outside of the cell to inside of the cell

Types of Membrane Proteins(2 of 2)

4. Recognition ID tags – glycolipids or glycoproteins

5. Intercellular Joining Join together adjacent cells

6. Support Attach to cytoskeleton or extracellular matrix

for stability

The phospholipid bilayer is __________ in the middle and __________ on the outsides.

1 2 3 4

25% 25%25%25%1. Hydrophobic, hydrophilic

2. Hydrophilic, hydrophobic

3. Hydrophobic, hydrophobic

4. Hydrophilic, hydrophilic

What is the function of cholesterol in the cell

membrane?

1 2 3 4

25% 25%25%25%1. Identification2. Transport3. Membrane

fluidity4. Recognition

The picture shows a peripheral protein.

1 2

50%50%1. True2. False

Which of the following is NOT a function of

membrane proteins?

1 2 3 4

25% 25%25%25%1. Signal transduction

2. Identification3. Transport4. Support

Passive Transport Passive Transport – the movement

across the cell membrane that does not require energy

Types: Diffusion

Osmosis

Facilitated Diffusion

Diffusion Diffusion – the movement of particles from an

area of high concentration to an area of low concentration until an equilibrium is reached

Transports small, nonpolar molecules such as CO2 and O2

Concentration Gradient – the difference in the concentration of a substance across a space

Equilibrium – concentration of a substance is equal throughout a space

Osmosis Osmosis – diffusion of free water across a

selectively permeable membrane

Water diffuses across the cell membrane from the region of low solute concentration to that of a higher solute concentration until it reaches equilibrium

U-Tube (not YouTube!)

Which way will water flow?

U-Tube (not YouTube!)

This way!

Osmoregulation in Cells Without Cell Walls

Osmoregulation: control of water balance

Tonicity: the ability of a solution to cause a cell to gain/lose water Isotonic solution: no net movement of water across

the cell membrane

Hypertonic solution: more free water inside the cell (cells lose water & shrivel)

Hypotonic solution: less free water inside the cell (cells gain water & lyse)

Some cells have a contractile vacuole to pump water out of the cell

http://www.youtube.com/watch?v=mTXRcbjuYGU

Osmoregulation of Cells With Cell Walls

Plant cells are healthiest in a hypotonic solution Osmotic pressure keeps cell walls turgid (very

firm)

Plant cells are flaccid (limp) in an isotonic solution

In a hypertonic solution, the cell membrane will shrink and pull away from the cell wall AKA plasmolysis (wilting)

http://www.youtube.com/watch?v=EsJ73x4ycp0

Facilitated Diffusion Facilitated Diffusion: passive transport aided

by proteins

Molecules travel from high low concentration

Transports small, polar molecules

Glucose

Salts

Ions

Amino Acids

Water (aquaporins)

Channel Proteins Hydrophilic passageways

Some are always open for diffusion

Rate of movement is determined by the concentration gradient

Some ion channels have gates that can be opened by: Cell membrane stretching

Change in electrical charge

Binding of specific molecules

Carrier Proteins

1. Specific substance binds to carrier protein

2. Protein changes shape & transports substances across the cell membrane

3. Molecule is released into the cell, and carrier protein returns to its original shape

http://www.youtube.com/watch?v=GFCcnxgXOhY

Active Transport

Uses energy to move solutes AGAINST the concentration gradient

Carrier proteins act as “pumps” powered by ATP

Examples: Sodium Potassium Pump

Proton Pump

Cotransport

Sodium Potassium Pump

Pumps 3 Na+ out of the cell and 2 K+ into the cell

Actively transports both ions against their concentration gradient

Powered by ATP

http://www.youtube.com/watch?v=GTHWig1vOnY

Proton Pump Actively transport protons (H+) through the internal

membranes of mitochondria & chloroplasts

Cotransport (Coupled Transport)

Cotransport: The transport of one solute indirectly drives the transport of several other solutes (using ATP)

Example: As proton pump pumps H+ out, H+ ions diffuse back in pulling sucrose with it

Bulk Transport

Substances that are too large to be transported by carrier proteins

Example: proteins leaving/entering the cell

Uses vesicles

Types: Exocytosis

Endocytosis

Exocytosis

Exo = Export (recall exo means outside!!)

Vesicles fuse with the cell membrane, releasing the contents outside of the cell

Endocytosis Recall endo means inside!!

Cell membrane engulfs particles

Pinches off to form vesicles inside of the cell

http://www.youtube.com/watch?v=ygkvXT1BmaA